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MG MGB Technical - O2 sensor optimal location

Previously I was told the following is not possible. Love this board's take on it.

I'd like to put a wide band O2 sensor in. However, I would like a dual reading to accurately see what each carb is doing and naturally to better sync them A/F wise.

The stated place is where the muffler header converges into a single tube for the silencers sections. Is there anyway to put them in a place where I can monitor both SU's? Using something like this:


with a standard cast exhaust manifold, no! The closest point would be in the exhaust down pipes by which time the output of cyl 1&4 will be mixed. cyl 2&3 get mixed in the head.
If you have a tube manifold you could try one in each branch of the Y but I'm thinking the gas temp might be a bit on the high side for the sensors to last long. Do they have a max operating temp?

best of....
M McAndrew

Bummer. This is from their site on temp.

The maximum temperature of the sensor at the bung (the sensor mounting location) should not exceed 500 oC or 900 oF. If these temperatures are exceeded in your application you should install the Innovate Motorsports HBX-1 heat sink bung extender. (p/n 3729.)

Max some racers use manifold temperature readings from 1 and 4, close to the head for the mixture settings. Far too hot for A/F sensors but useful for full power. I use a PLX A/F at the merge and have found it great. Denis
D M Hill

Thanks Denis for the education and confirmation.

With the sensor at the junction how to you find it helps basic fuel sync? I'm good at sync with air intake, but that final trick would be great. I look at the plugs and raise the piston etc, but I know its not perfect by any means.

A/F meter actually solved a problem I had with the car's rebuild for 4 years so I'm sold on the concept.

My car is supercharged so only one carburetor. I fitted the PLX several years ago when playing around looking for performance gains and safe mixtures. Its semi permanent in the car, however I do remove it occasionally doing other cars. With multi carbs the main thing is to get them all the same first and one way is to loosen the joining links, close the butterflies off and do one carb at a time. Then once set the same, re connect the linkage and set the idle speed and mixture, making the same adjustment on all carburetors. Some times all that's necessary Is to visually set the jet heights the same and balance the air flow but I try to adjust the carbs the same amount equally so the PLX in not a average of one lean and one rich. I adjust SUs "mixture" for maximum revs at idle and find that if the needles are correct the other areas, throttle response, power and cruise fall into place A big mistake with SUs is is a lean idle and I aim for an idle in the 12:1 area (or a little richer with the supercharger. ) Denis
D M Hill

Sorry to be a little dense. Won't closing the butterfly on one stall the engine? Or is it short enough time that it work?

I do check the jet height with a micrometer although they still seem to need a little independent adjusting from there....

The average as you described is what I want to avoid..

I fitted an O2 sensor. It will tell you what the mixture is very (too) accurately. Then what do you do? Either adjust he jets which changes the mixture throughout the range and for which you really don't need a Lambda sensor. Or try to adjust positions on the needles. The difficulty is identifying which station to adjust. Doable, tho for testing purposes you really need a rolling road. I decided that whatever changes I could make would make little overall difference. Fine as an interesting project but I would not expect much "improvement " overall.
Michael Beswick

Sorry missed a bit........ much improvement over using the conventional methods.
Michael Beswick

You open the one your checking first say two turns, then close exactly it the same amount before checking the other carb. You only need to do this initially and its a good way to make sure your settings are at least the same for the A/F reading as a "starting point". Having done this make the same adjustments to each carb for idle speed and mixture. Denis
D M Hill

This thread was discussed between 01/06/2017 and 02/06/2017

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